1. Field of the Invention
The present invention relates to a color cathode-ray tube, and more particularly, to a mask-frame assembly having an improved frame for supporting a mask to apply tension in a color cathode-ray tube.
2. Description of the Related Art
In typical color cathode-ray tubes, three electron beams emitted from an electron gun pass through beam passage holes of a shadow mask for color selection and land on red, green and blue phosphors of a phosphor layer formed on the screen of a panel to excite the phosphors, thereby forming an image. In such color cathode-ray tubes, dot masks employed in computer monitors and slot (or slit) masks employed in televisions are representative masks for color selection. Such a mask is fixed to a frame and installed within a panel of a cathode-ray tube.
Representative slot masks are forming masks which are designed to have a curvature corresponding to the curvature of a screen taking into account landing of electron beams deflected with respect to the screen and tension masks which are supported such that tension is applied thereto taking into account a screen which is flattened to compensate for image distortion and expand a visual field angle.
Structures of a combination of a frame and a mask to which tension is applied are disclosed in Japanese Patent Publication Nos. sho 59(1984)-18825 and sho 59(1984)-16626. The disclosed tension mask-frame assembly includes a pair of support bars which are disposed in parallel to each other with a predetermined space therebetween, and substantially U-shaped resilient support members for supporting both ends of the support bars. An aperture grill-type mask is fixed to the support bars such that a tension can be applied to the mask.
In a course of manufacturing the above mask-frame assembly, a blackening process and an annealing process are performed to eliminate a stress attendant upon welding of the support bars and resilient support members and to blacken the mask and the frame composed of the support bars and the resilient support members. During the above processes, the mask-frame assembly is heated to about 500 C. Here, plastic deformation or thermal creep occurs in the mask due to the difference in the amount of thermal expansion between the frame and the mask and a decrease in a breaking strength limit depending on temperature, thereby causing a problem of a reduction in the tension (by 20%). In other words, when the mask-frame assembly is heated, a difference in the amount of thermal expansion between the mask and the frame occurs because the heat capacity of the mask is smaller than that of the frame. The difference in the amount of thermal expansion acts on the mask supported by the support bars as an additional tension, thereby decreasing the tension of the mask after the blackening and annealing processes. The decrease in the tension of the mask causes howling or drift of electron beams when a color cathode-ray tube employing the mask-frame assembly is place in operation.
To overcome the above problem, U.S. Pat. No. 5,111,107 discloses a mask-frame assembly for preventing the expansion of a frame from acting in the tension direction of a mask. The disclosed mask-frame assembly is shown in FIG. 1. The mask-frame assembly includes mutually opposed support bars 11, resilient support members 12 attached between the support bars to support the support bars, a mask 13 supported by the support bars 11, and metallic members 14 which have a larger thermal expansion coefficient than the resilient support members 12 and are connected to the lower surfaces of the resilient support members 12 opposite to the surfaces thereof facing the mask 14. Even though the mask-frame assembly 10 is provided with metallic members 14, the tension of the mask 13 is decreased, and the effect of the metallic members 14 varies with a tension distribution.
Meanwhile, Japanese Patent Publication No. hei 11-317176 discloses a color cathode-ray tube having a mask-frame assembly for reducing a decrease in the tension of a mask during the blackening and annealing processes. In the disclosed color cathode-ray tube, a color selection electrode is installed at a frame which includes a pair of mutually opposed supports and a pair of resilient support members disposed between the supports. Adjustment members having a smaller thermal expansion coefficient in a low temperature range and a greater thermal expansion coefficient in a high temperature range than the resilient support members are fixed to the sides opposite to the sides of the resilient support members facing the grid, or adjustment members having the reverse characteristic of the above are fixed to the sides of the resilient support members . A color selection device of a color cathode-ray tube having such a structure, in which adjustment members are attached to resilient support members to use a difference in thermal expansion coefficient as described above, cannot radically overcome the above problems.
A different color selection device and color cathode-ray tube are disclosed in Japanese Patent Publication No. 2000-3682. The disclosed color selection device includes a color selection electrode assembly with a mask suspended on a frame. A tension adjustment member parallel to the mask of the color selection electrode assembly is installed at a color selection electrode frame. Such a color selection device including a tension adjustment member installed at a frame to compensate for a decrease in a tension after a blackening or annealing process cannot prevent a decrease in the tension of a mask during the blackening or annealing process.